DESCRIPTION (provided by investigator): Recent studies have revealed the presence of self-renewing intestinal stem cells (ISCs) in the midgut of the fruit fly, Drosophila melanogaster. As in the human colon and small intestine, these stem cells allow dynamic self-renewal of the fly's intestinal epithelium. The relative simplicity of the fly midgut, and the superlative genetic tools available, make this a promising system for studies of stem cell biology and gut homeostasis. This proposal addresses the mechanism of homeostatic growth in the fly intestine, with the goal of understanding the genetic and environmental control of intestinal stem cell proliferation. Our first aim is to define the essential cell-cell interactions that mediate gut homeostasis. To do this we will alter rates of cell death and proliferation in the four major midgut cell types (stem cells, enterocytes, enteroendocrine cells, visceral muscle) and then assess the effects on ISC proliferation, differentiation, and total intestinal cell numbers. Our second and third aims apply genetic and cytological approaches to probe the functions of two genes that cause intestinal stem cell hyperplasia in the fly. One gene, Unpaired, encodes a leptin-like ligand for JAK/STAT signaling and the other, Ack, encodes a non-receptor tyrosine kinase similar to Src. Human homologs of both have been implicated as causal agents in cancers, but their function and potential interaction is poorly understood for the intestine. Finally, we propose two screens, one genetic and one molecular, to identify novel genes that regulate intestinal stem cell proliferation and/or differentiation. The availability of intestinal stem cell markers and excellent genetic tools in Drosophila will allow this project to progress rapidly. The structural and regulatory similarities between insect and mammalian intestines suggest that the yield will be relevant to gut homeostasis in humans and, potentially, to the causes of colorectal cancer (CRC), the second leading cause of cancer deaths in the U.S. Colon cancer arises from mutations in intestinal stem cells that allow their autonomous proliferation. Although several genetic targets that promote CRC progression are known, there are multiple routes to CRC, and key genes in the process remain to be identified. This project may help to identify these important genes. Public Health Relevance: This project uses genetics to address the regulation of intestinal stem cell proliferation in the fruit fly, Drosophila melanogaster. The similarities between mechanisms of gut homeostasis in flies and man suggest that some of the genes and regulatory interactions discovered herein may be relevant to human diseases that involve deregulated stem cell growth. For instance, these studies could provide new biomarkers and gene targets that are eventually used for the diagnosis and treatment of colorectal cancer.